Four wheel drive vehicles having a transfer case in the driveline for distributing power to the front and rear drive axles are known in the art. In such vehicles, the transfer case is usually provided with two or more output shafts which are driven by a main or input shaft. The driven shafts may be referred to as output drive shafts since they are used to drive the vehicle road wheels through the drive axles. Some differential in the speed between the shafts is necessary to permit different rotational speeds of the driving wheels to accommodate vehicle steering. It is known to couple these output shafts by means of a differential. In some applications, a bevel gear differential, which evenly splits the torque between the drive axles, is used in the transfer case to drive the front and rear axles at all times, yet allow relative rotation between the axles to accommodate steering geometry. The use of a gear differential in a drive train has one serious drawback. That is, if any road wheel of the vehicle is on a low traction surface, the various axle and transfer case differentials allow that wheel to turn freely. As such, little power or torque is delivered to the remaining wheels.
To minimize wheel slippage, the transfer case differential is sometimes equipped with a manually operated lock-up mechanism. Such a mechanism is operated in either a locked or unlocked condition. When locked, such a mechanism connects the front and rear drive shafts together and positively drives them both. Such a locking mechanism does not allow, however, any differentiation between front and rear drive axle turning speeds.
Several systems have been devised to automatically shift a vehicle from two wheel drive to four wheel drive. With such systems, the front and rear drive wheels are locked together upon the detection of wheel slip. Those systems which automatically shift between two and four wheel drive have several drawbacks. First, such systems do not offer full-time four wheel drive. Thus, the improved vehicle handling and safety characteristics obtainable with full time four wheel drive cannot be achieved with such systems. Second, those systems which automatically shift between open or locked states lack flexibility. Once the system locks the front and rear drive wheels, no speed differential can be incurred therebetween. In many instances (i.e., cornering) it is desirable to shift torque from one drive shaft to the other while allowing speed differences between them. Furthermore, in those instances where a two wheel drive is automatically transmuted into four wheel drive in response to wheel slip, the automatic lock-up characteristic may cause the previously gripping set of wheels to loose traction. Thus, a need remains for a system wherein torque between front and rear drives may be shifted at something less than full lock-up.
There are also those systems which shift a four wheel drive vehicle to two wheel drive in response to a steering sensor. Again, such systems operate in either a locked or unlocked condition. Such systems do not afford the benefits of full-time four wheel drive. In many instances, including cornering, it is desirable to be able to control the torque split between the front and rear wheels. The heretofore known systems do not and cannot offer such abilities.